Assessment of chemical and bacteriological status of treated wastewater of the Baraki WWTP (Algeria) for agricultural purposes

Located in the south of the Mediterranean region, Algeria is among the poorest countries in terms of water potential which is below the theoretical scarcity threshold set by the World Bank at 1,000 m³/capita/year, despite the available water resources estimated to be 17 billion m³ (Touil et al. 2020). Algeria's water stress indicator has risen from 104.92 in 2010 to 137.2 in 2019; this is confirmed in terms of freshwater availability per capita, which fell from 314 m³/capita/year in 2010 to 263 m³/capita/year in 2019, a drop of around 16.24% in barely 10 years (Bakhtache & Hadjene 2023). The water crisis has affected the agricultural sector which is known to be the biggest freshwater consumer and irrigation needs are around 1.5 M ha. The agricultural sector consumes more than 60% of the mobilized water resources and it is planned to extend irrigated land to more than 1,560,000 hectares by 2030. The impact of climate change on agricultural production results from the combined effect of temperature and rainfall variability on crop yields. The new water policy is structured around two axes; the development of water infrastructure and institutional reform of the water sector to promote better management of the resource and good governance (MEER 2023). In order to alleviate this water stress in agriculture, recycling and reuse of treated wastewater (TWW) seems to be a desirable alternative and a water resources management tool arising from the need for a regulated supply that compensates for water shortages (Jaramillo & Restrepo 2017). The recovery of wastewater by-products (treated water and sludge) fits perfectly into the NEXUS concept: Water–Energy–Food. This concept consists of establishing the link between these three components and putting this approach into practice in order to ensure better interaction between the various sectors and achieve the objectives of water and food security. According to the national sanitation office, 220 wastewater treatment plants (WWTPs) are installed on Algerian territory with a capacity of 1.039 billion m³/year, treated volume of 540 million m³/year and reused volume of 59 million m³/year (11%). Only 15 of WWTPs are oriented toward the reuse of TWW in agriculture, representing a volume of 15.3 million m³, which represents 5.5% of the total volume treated for irrigating 10,125 ha of agricultural lands. These plants are situated in different areas; Ain El Houtz (Tlemcen), Azzefoun (Tizi Ouzou), Ammi Moussa (Relizane), Boumerdes, Guelma, Ain Temouchent, Mascara et Mohammadia (Mascara), Bouguirat, Hadjadj, Sidi Lakhdar, Khadra (Mostaganem), Timgad (Batna), El Bayedh and Medea (ONA 2024). The transport and reuse of TWW in agriculture are governed by executive decree 07-149 of 20 May 2007 (JORA N°35 2007).

If municipal wastewater is adequately treated, it can be used for irrigation which represents a valid alternative to conventional water resources. However, the most common problems that result from using TWW in irrigation are salinity, permeability, toxicity and miscellaneous (Ayres & Westcot 1985). Irrigating crops with this water may present several possible hazards to human health due to pathogenic microorganisms, heavy metals, and harmful organic chemicals that could be consumed or encountered (Shakir et al. 2017). The 2006 WHO guidelines for TWW use constitute a preventive management tool for agriculture as they provide guidance for decision-makers on the application of TWW in different local contexts. The objective of these guidelines is to support the formulation of government standards and regulations regarding the use and management of TWW, taking into account the specificities of each country (Mara et al. 2007). The Algerian standards for the reuse of TWW are based on Food and Agriculture Organization (FAO) and World Health Organization (WHO) directives which consist, in addition to physico-chemical parameters, of an assessment of particularly microbiological risks before any use, by collecting data on the existing pathogens. Therefore, it provides guidance on the management of urban waste and the prevention of health risks linked to the use of this resource in agriculture (JORA N°41 2012; Algerian standard 17683 2014). Although the use of TWW in agriculture for irrigation is an old practice in different countries, it has not been widely investigated and evaluated in Algeria. However, a study has been carried out on the CORSO station (Boumerdes- East Algiers) which is considered to be a pilot for vine irrigation with TWW. The results of analyses carried out on grape juice showed that grapes irrigated with TWW were contaminated with heavy metals, with levels varying from one element to another and from one variety to another. Considering that these values increased with subsequent irrigation and some elements exceeded the standards (Djillali 2020).

In view of these facts, the present study was undertaken to characterize the TWW produced from the WWTP of Baraki (Algiers), which used sand filtration as tertiary treatment, and to evaluate its suitability for irrigation purposes as non-conventional water resources according to national (JORA N°41 2012; Algerian standard 17683 2014) and international (FAO 2003; WHO 2006) standards. Also, in order to support the new national policy on wastewater treatment, our contribution takes into account the identification of any failures encountered and their repercussions on the usefulness of wastewater treatment stations. Sharing this expertise with the managers of the various stations set up at the national level will enable the objectives of this new policy to be met in the best possible way.

Daily TWW samples were automatically collected using auto-samplers in one-liter polyethylene bottles for the measurement of pH, conductivity (EC), total suspended solid (TSS), biological oxygen demand (BOD₅), chemical oxygen demand (COD), and weekly samples for total nitrogen (TN), ammonia (⁠⁠), nitrate (⁠⁠), and total phosphate (TP). The macronutrient concentrations of calcium (⁠⁠), magnesium (⁠⁠), sodium (Na⁺), potassium (K⁺), carbonates (⁠⁠), and bicarbonates (⁠⁠) were analyzed once a month, while heavy metals were measured twice a year. The samples collected in polyethylene bottles were stored at a temperature below 4°C prior to analysis. For microbiological analyses, samples were collected twice a month in sterile glassware containers and transported to the laboratory under refrigeration. The period studied is from January to December 2022.

The electrical conductivity (EC) and pH were determined by digital electrochemistry HQ40D multi (HACH). The TSS was evaluated by filtration on a glass fiber filter and the filter was dried at 103–105 °C. The BOD₅ was measured by the Oxitop method after 5 days of incubation, while COD, ⁠, ⁠, TN, and TP were determined by using the LCK cuvette test method (HACH) and reading the results by spectrophotometer LT200 LANGE. The concentrations of Ca²⁺ and Mg²⁺ were quantified titrimetrically using standard EDTA. The carbonates and bicarbonates were measured titrimetrically using hydrochloric acid. The ICP-OES (OPTIMA 8000) was used to quantify sodium, and potassium and to detect the traces of elements such as; cadmium (Cd), copper (Cu), zinc (Zn), lead (Pb), chromium (Cr), arsenic (As), nickel (Ni), manganese (Mn), iron (Fe), selenium (Se), antimony (Sb), silver (Ag), barium (Ba), molybden (MO), boron (B), mercury (Hg), and aluminum (Al).

For the bacteriological quantification, the samples were dissolved in sterile saline water for dilution and all measurements were performed by membrane filtration method (Rodier 2009). Total coliforms (TC), fecal coliforms (FC), Escherichia coli (E. coli), fecal enterococci (FE), pathogenic germs of the genus Salmonella spp. and Staphylococcus spp. were identified and counted according to the French national standard (ISO). The results were expressed as number of colonies forming units (CFU) per 100 mL.

The collected records have all been abridged into statistical values which are presented in the following form; average ± standard deviation. Normal distribution was tested by Kolmogorov–Smirnov test and non-parametric tests were used in case of non-normal distribution. The effect of the season upon quantitative parameters was tested using Friedman repeated measures analyses of variance on ranks and comparisons between seasons were performed using the Wilcoxon test. Differences were considered to be statistically different at p < 0.05. SPSS (version 26; INC., Chicago, IL, USA) was used for statistical evaluation of the results.

The analysis results of effluent from Baraki WWTP are presented in Table 1, interpreted and compared to quality recommendations and standards.

Table 1

Summary statistics of the analytical data from January 2022 to December 2022

Parameter .	Unit .	Mean .	Standard deviation .	Standard error .	Max .	Min .	FAO standard .
pH		7.85	0.25	0.03	8.1	7.16	6.5–8.5
EC	μS/cm	1,511	118	14.46	1,708	1,341	3,000
TSS	mg/L	21.84	12.29	1.57	46.4	7.23	30
COD	mg/L	56.08	31.85	3.94	122.93	25.69	90
BOD	mg/L	15.94	16.91	2.23	53.08	4.1	30
TN	mg/L	57.19	20.63	5.55	97.1	35.95	30
	mg/L	28.55	10.37	2.79	41.3	13.43	–
	mg/L	3.67	1.68	0.45	6.6	1.55	30
TP	mg/L	3.43	2.21	0.59	7.45	1.39	–
Ca2+	mg/L	95.25	5.53	3.13	109	90	–
Mg2+	mg/L	26.25	2.34	1.32	29	21	–
Na+	mg/L	111.58	17.4	9.84	146	86	–
K+	mg/L	16.33	1.72	0.97	18	13	–
	mg/L	348.33	60.38	34.16	429	199	518.5
	mg/L	0	0	0	0	0	–
Cd	μg/L	<2	0	0.00	<2	<2	10
Cu	μg/L	29	0	0.00	40	17	200
Zn	μg/L	22	0.01	0.01	30	13	2,000
Pb	μg/L	<2	0.01	0.01	<2	<2	5,000
Cr	μg/L	18	0.01	0.01	23	13	100
As	μg/L	<2	0.01	0.01	<2	<2	100
Ni	μg/L	14	0.01	0.01	21	6	200
Mn	μg/L	56	0.02	0.03	77	34	200
Fe	μg/L	625	0.09	0.12	711	538	5,000
Se	μg/L	<1	0	0.00	<1	<1	20
Sb	μg/L	<2	0	0.00	<2	<2	–
Ag	μg/L	17	0.1	0.14	31	<2	–
Ba	μg/L	65	0	0.00	69	61	–
Mo	μg/L	<2	0	0.00	<2	<2	10
B	μg/L	147	0.14	0.19	288	5	750
Hg	μg/L	<2	0	0.00	<2	<2	–
Al	μg/L	115	0.11	0.15	255	5	5,000

Parameter .	Unit .	Mean .	Standard deviation .	Standard error .	Max .	Min .	FAO standard .
pH		7.85	0.25	0.03	8.1	7.16	6.5–8.5
EC	μS/cm	1,511	118	14.46	1,708	1,341	3,000
TSS	mg/L	21.84	12.29	1.57	46.4	7.23	30
COD	mg/L	56.08	31.85	3.94	122.93	25.69	90
BOD	mg/L	15.94	16.91	2.23	53.08	4.1	30
TN	mg/L	57.19	20.63	5.55	97.1	35.95	30
	mg/L	28.55	10.37	2.79	41.3	13.43	–
	mg/L	3.67	1.68	0.45	6.6	1.55	30
TP	mg/L	3.43	2.21	0.59	7.45	1.39	–
Ca2+	mg/L	95.25	5.53	3.13	109	90	–
Mg2+	mg/L	26.25	2.34	1.32	29	21	–
Na+	mg/L	111.58	17.4	9.84	146	86	–
K+	mg/L	16.33	1.72	0.97	18	13	–
	mg/L	348.33	60.38	34.16	429	199	518.5
	mg/L	0	0	0	0	0	–
Cd	μg/L	<2	0	0.00	<2	<2	10
Cu	μg/L	29	0	0.00	40	17	200
Zn	μg/L	22	0.01	0.01	30	13	2,000
Pb	μg/L	<2	0.01	0.01	<2	<2	5,000
Cr	μg/L	18	0.01	0.01	23	13	100
As	μg/L	<2	0.01	0.01	<2	<2	100
Ni	μg/L	14	0.01	0.01	21	6	200
Mn	μg/L	56	0.02	0.03	77	34	200
Fe	μg/L	625	0.09	0.12	711	538	5,000
Se	μg/L	<1	0	0.00	<1	<1	20
Sb	μg/L	<2	0	0.00	<2	<2	–
Ag	μg/L	17	0.1	0.14	31	<2	–
Ba	μg/L	65	0	0.00	69	61	–
Mo	μg/L	<2	0	0.00	<2	<2	10
B	μg/L	147	0.14	0.19	288	5	750
Hg	μg/L	<2	0	0.00	<2	<2	–
Al	μg/L	115	0.11	0.15	255	5	5,000

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As described in the literature, the pH is an important parameter to assess in the monitoring of TWW quality for agriculture purposes. Thus, a pH varied from 6.5 to 8.4 is considered normal and acceptable for irrigation. However, if the pH was found outside this range, it means that the TWW may contain toxic ions and induce a nutritional imbalance (Pescod 1992; Alobaidy et al. 2010; Shakir et al. 2017). Our pH values ranged between 7.16 and 8.10 with a mean value of 7.85 ± 0.25, slightly alkaline but in the range of normal pH for irrigation water (6.5–8.4). This TWW is suitable for irrigation and may not cause a negative effect on plants and soil in terms of toxic ions.

Salinity is a key parameter in determining the suitability of the TWW to be used for irrigation and the wide variability of salinity tolerance in plants can confound the issue of establishing salinity criteria (EPA 2004). Salinity is assessed by determining EC which measured all dissolved anions and cations in water. The EC values of our experimental samples varied from 1,341 to 1,708 μS/cm (mean value = 1,511 ± 118 μS/cm). Based on the classification of the USSL, irrigation water is classified as high salinity class C3 of 750–2,250 μS/cm, permissible for irrigation but it cannot be used on poorly drained soils even when the drainage is sufficient (Richards 1954). Our obtained EC values may indicate a moderate level of restriction for the usage of TWW in irrigation because of adding salt concentration to the soil which increases the initial concentration, thus causing adverse effects on crops. In fact, in the case of high EC, the plant's osmotic activity will be reduced creating interferences with the absorption of nutrients and water from the soil (Tatawat & Chandel 2008). Generally, in similar situations, salinity monitoring management with the selection of salt-tolerant plants is recommended (Alobaidy et al. 2010).

The TSS represents the totality of the insoluble mineral and organic particles floating or suspended that are contained in wastewater (Nakib et al. 2016). TSS also contributes to soil fertilization through its richness in organic matter. The average TSS content encountered was 21.84 ± 12.29 mg/L. It is less than standards (30 mg/L) thus allowing their use in irrigation without the need of filtration (FAO 2003).

COD is the quantity of oxygen consumed by the matter in the water (Rodier 2009). Higher COD indicates a higher content of oxidizable matter in the water, thus water will have reduced dissolved oxygen levels (Shirin & Yadav 2014). Biochemical Oxygen Demand (BOD₅) expresses the amount of dissolved oxygen needed by aerobiological organisms in a body of water to break down organic matter (Moghadam et al. 2015). If BOD₅ is low, the water has a low content of organic matter and also low counts of microbial organisms. If BOD is high, the water has higher organic matter and this will reduce the overall quality of water (Poonia et al. 2023). The average concentrations of the biochemical oxygen demand (BOD₅) and COD are 15.94 ± 16.91 mg/L and 56.08 ± 31.85 mg/L, respectively. The TWW in this study area displayed less values compared to FAO recommendations which require BOD₅ < 30 mg/L and COD < 90 mg/L.

The ratio COD/BOD₅ represents the ability to biodegradability of the effluent and depends on the nature. For our TWW, we noted an average value of 4.95 ± 1.92 which is superior to 3 indicating moderately biodegradable effluent (Rodier 2009). This shows that the contribution of industrial activities to change both BOD₅ and COD is high (Tamrabet 2011).

Moreover, the ratio BOD₅/COD gives a very interesting indication about the origin of pollution (Karef et al. 2017). The calculation results ranged from a minimum of 0.12 to a maximum of 0.43 with an average of 0.24. This value is lower than that found by other researchers who reported 0.56 (Karef et al. 2017). Therefore, this effluent is moderately biodegradable and confirms that these waters are loaded with organic matter (26%) and inorganic matter (74%). These results indicate that the wastewater needs more treatment.

The nutrients contained in the TWW constitute an important quality parameter for the valorization of this water in agriculture and landscape management. The TWW containing these elements; N and P can act as a fertilizer for plants, therefore it is possible to reduce the use of chemical fertilizers (Moghadam et al. 2015). However, it should be noted that it may also contaminate the aquifer with nitrates in case of high concentrations (Rahimi et al. 2018). One of the essential stages in wastewater treatment is the elimination of nitrogen. The biogeochemical evolution of organic nitrogen leads to the formation of ammonia, nitrites and nitrates (Bachi et al. 2022). The results of analyzing the studied Baraki's WWTP effluent showed low and permissible amounts of nitrate (⁠⁠) of 3.43 ± 2.21 mg/L. However, it was noted a very high ammonia (⁠⁠) content averagely 28.55 ± 10.37 mg/L, exceeding the permissible limit of USEPA (10 mg/L). Our found ammonia (⁠⁠) concentration exceeds the value of 2.28 mg/L mentioned by (Djemil et al. 2018) in TWW from WWTP of Baraki (Algiers) and 3.55 mg/L mentioned by (Djillali et al. 2020) in TWW from Corso WWTP (Boumerdes). TN content averagely 57.19 ± 20.63 mg/L which is higher than the permissible limit of FAO for irrigation (30 mg/L) and higher than the average content 15.31 mg/L found by Djillali et al. (2020). The value of TP is an average 3.67 ± 1.68 mg/L which is greater than the value 1.78 mg/L found by Djemil et al. (2018). This concentration does not exceed the limit recommended by Müller & Cornel (2017) of 13 mg/L, bearing in mind that this limit is based on the requirements of most crops.

Based on the recommendation of FAO to evaluate the specific toxicity of ions; boron is considered the most phytotoxic ions and concentrations above 500 μg/L can be toxic to sensitive crops. The mean value of boron in the TWW is 147 μg/L; inferior to the FAO standards fixed for long term use 750 μg/L. The heavy metals interesting to plants and pose health problems are Cu and Zn; Cu is toxic to a number of plants at 0.1–1.0 mg/L in nutrient solutions and zinc is toxic to many plants at widely varying concentrations (EPA 2004). The respective values of Cu and Zn were 29 and 22 μg/L. These contents are very negligible and have no toxic effects on either the ground or on the plant (200 and 2,000 μg/L, respectively for limit of long term use). The average value of Mn was 56 μg/L which is inferior to the standard (5,000 μg/L). The average values of Pb, Cd, Cr and Ni content were <2, <2, 18 and 14 μg/L, respectively. These results were found in a safe range to use in agricultural irrigation. The values of Pb, Cd and Cr found by Djemil et al. (2018) in the same WWTP studied are, respectively, 4,250, <50, 200 μg/L, which is superior to our values. The average values of As, Fe, Se, Mo and Al content were <2, 625, <1, <2 and 115 μg/L, respectively, these values are below the agricultural reuse standard. Regarding chemical compounds in TWW, heavy metals were in fewer concentrations compared to the FAO standards for irrigation water. Therefore, this source of water can be used for irrigation purposes without any hazardous effect on soil and plants, in terms of the absence of metal toxicity.

Table 2 groups together the indicators of water quality for irrigation, namely: SAR, SSP, and RSC.

The SSP is also another important parameter used to determine sodium hazard. The measured SSP value of the TWW ranges from 34.81 to 46.51% (mean = 39.69 ± 3.69). It is in accordance with the recommended mean value by the US Salinity Laboratory (40–60%), that indicates a moderate degree of restriction on the use of this wastewater in irrigation. Furthermore, TWW with an SSP higher than 60% can induce an accumulation of sodium in the soil and thus disrupt its physical properties. In fact, an ion exchange will be established between Na⁺, which is in excess in the ground, and Ca²⁺ and Mg²⁺ in clay particles. Unfavorable physical conditions result when sodium is the predominant cation (Wilcox 1948). This phenomenon will reduce the permeability and slow drainage and water infiltration in soil. Alobaidy et al. (2010) have demonstrated a positive correlation between SSP and SAR.

A high concentration of bicarbonates in water causes the precipitation of calcium and magnesium as carbonates. The RSC is calculated to qualify this effect (Tatawat & Chandel 2008). Higher values of RSC indicate high levels of water alkalinity, which affects irrigation (Badr et al. 2023). The water is not suitable for irrigation when the RSC value is greater than 2.5; while when it is between 1.25 and 2.5 the water may be considered on the critical side. The only safe region is when the RSC value is less than 1.25 (Alobaidy et al. 2010). The calculated RSC mean value of −1.20 ± 0.96 meq/L is less than zero indicating that this value is safe for irrigation and there is no complete precipitation of calcium and magnesium.

Wastewater is a very complex matrix containing organic, inorganic and biological compounds with different degrees of toxicity, depending on intrinsic parameters such as the variation of physical, chemical and microbiological properties and extrinsic parameters as environment, rainfall and temperature (cold and hot seasons). Seasonal monitoring of some parameters of TWW revealed significant differences, between seasons, for more of them (Table 3). To better understand if the weather conditions have an impact on this variation, the evaluation of temperature and rainfall has been done and presented in Table 4. The mean values of assessed parameters presented on Table 3 demonstrated that pH, EC, TSS, COD, BOD, and TP were significantly different (p < 0.05) between seasons of the year. However, no significant difference was noted for TN, ⁠, and the assessed cations Ca²⁺, Mg²⁺, Na⁺ and K⁺. Moreover, it was observed higher values of EC, TSS, COD, and BOD in winter with an average of low temperature and rainfall of 9.85 °C and 2.52 mm, respectively, compared to spring, summer and autumn. For the nutrients, the values of TN, and TP were also higher in winter, but was higher in summer coinciding with the higher values of pH, and cations. These results may be mainly linked to the variation in pollutant loads during the year, affected by diverse human activities, lifestyle, both municipal and industrial wastes, treatment processes, etc. Table 4 shows a normal and moderate variation in temperature during the year but low average rainfall with apparent fluctuation between winter, spring and autumn seasons. Numerous studies demonstrated that the climate changes, have significant effects on WWTP processes, manifested by increased both wastewater temperatures and inflow rates (Ranieri et al. 2023). The increase of temperatures influenced oxygen solubility, microbial metabolism, and biodegradation processes. Also, it compromised the biological oxygen demand (BOD) and therefore will cause harmful effects to aquatic ecosystems and communities (Ranieri 2003; Hughes et al. 2021).

Table 3

Seasonal variation of TWW parameters

Parameter .	Unit .	Winter .	Spring .	Summer .	Automne .
pH		8.02 ± 0.13a	7.87 ± 0.25b	8.09 ± 0.12a	7.9 ± 0.17b
EC	μS/cm	1,617.56 ± 206.22a	1,472.54 ± 169.88b	1,517.09 ± 62.28b	1,458.23 ± 97.10c
TSS	mg/L	26.43 ± 21.42a	19.57 ± 15.22b	19.03 ± 19.38b	16.8 ± 19.87c
COD	mg/L	81.75 ± 43.44a	60.29 ± 37.07b	37.70 ± 13.08c	36.70 ± 10.90c
BOD5	mg/L	25.80 ± 23.64a	21.55 ± 21.63a	9.48 ± 5.34b	4.92 ± 1.93c
TN	mg/L	73.40 ± 40.44a	53.26 ± 16.50a	50.95 ± 18.23a	51.25 ± 36.61a
	mg/L	28.52 ± 17.00a	31.48 ± 13.01b	36.29 ± 6.46b	19.44 ± 7.95c
	mg/L	4.51 ± 5.34a	3.57 ± 2.54a	2.46 ± 1.25a	3.84 ± 1.58a
TP	mg/L	6.34 ± 5.76a	4.36 ± 2.76a	2.45 ± 1.84b	2.04 ± 1.71c
Ca2+	mg/L	91.67 ± 2.08a	93.33 ± 1.15a	101.00 ± 7.00a	95.00 ± 6.24a
Mg2+	mg/L	25.33 ± 0.58a	25.33 ± 4.04a	27.67 ± 2.31a	26.67 ± 1.53a
Na+	mg/L	98.00 ± 11.53a	135.00 ± 16.52a	109.00 ± 5.19a	104.33 ± 6.65a
K+	mg/L	14.33 ± 2.31a	17.33 ± 0.58a	17.33 ± 1.15a	16.33 ± 0.58a
	mg/L	316.33 ± 115.07a	354.67 ± 28.18a	364.67 ± 56.01a	359.67 ± 26.27a

Parameter .	Unit .	Winter .	Spring .	Summer .	Automne .
pH		8.02 ± 0.13a	7.87 ± 0.25b	8.09 ± 0.12a	7.9 ± 0.17b
EC	μS/cm	1,617.56 ± 206.22a	1,472.54 ± 169.88b	1,517.09 ± 62.28b	1,458.23 ± 97.10c
TSS	mg/L	26.43 ± 21.42a	19.57 ± 15.22b	19.03 ± 19.38b	16.8 ± 19.87c
COD	mg/L	81.75 ± 43.44a	60.29 ± 37.07b	37.70 ± 13.08c	36.70 ± 10.90c
BOD5	mg/L	25.80 ± 23.64a	21.55 ± 21.63a	9.48 ± 5.34b	4.92 ± 1.93c
TN	mg/L	73.40 ± 40.44a	53.26 ± 16.50a	50.95 ± 18.23a	51.25 ± 36.61a
	mg/L	28.52 ± 17.00a	31.48 ± 13.01b	36.29 ± 6.46b	19.44 ± 7.95c
	mg/L	4.51 ± 5.34a	3.57 ± 2.54a	2.46 ± 1.25a	3.84 ± 1.58a
TP	mg/L	6.34 ± 5.76a	4.36 ± 2.76a	2.45 ± 1.84b	2.04 ± 1.71c
Ca2+	mg/L	91.67 ± 2.08a	93.33 ± 1.15a	101.00 ± 7.00a	95.00 ± 6.24a
Mg2+	mg/L	25.33 ± 0.58a	25.33 ± 4.04a	27.67 ± 2.31a	26.67 ± 1.53a
Na+	mg/L	98.00 ± 11.53a	135.00 ± 16.52a	109.00 ± 5.19a	104.33 ± 6.65a
K+	mg/L	14.33 ± 2.31a	17.33 ± 0.58a	17.33 ± 1.15a	16.33 ± 0.58a
	mg/L	316.33 ± 115.07a	354.67 ± 28.18a	364.67 ± 56.01a	359.67 ± 26.27a

The difference between superscripts letters in the same line indicates a statistical differences (p < 0.05).

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Microbial examination of TWW revealed a high concentration of indicator bacteria. The TC content is averagely 2.E + 06 UFC per 100 mL which is higher than the permissible limits in WHO standards which recommended that TWW intended for crop irrigation should contain less than 1,000 FC bacteria per 100 mL (Table 5). The mean concentration of E. coli of 5.E + 05 UFC per 100 mL was much higher than the amount recommended by the WHO for wastewater reuse in irrigation for root and leafy crops (10³ and 10⁴E. coli per 100 mL for root and leafy crops, respectively) indicating that wastewater irrigation could increase the risk of enteric pathogen infections in consumers of wastewater irrigated crops, even for high-growing crops (WHO 2006). The content of FE is averagely 4.E + 05 UFC per 100 mL, which reflects a high fecal pollution load in water due mainly to domestic sewage. However, pathogen analysis showed negative results for all samples analyzed. So, Salmonella spp. and Staphylococcus spp. were not found in TWW during the study period due to absence at the station entrance or to the removal of microorganisms by treatment processes. A number of studies have reported no detection of Salmonella in treated municipal wastewater (Cirelli et al. 2012; Gatta et al. 2016; Farhadkhani et al. 2018). A high concentration of bacteria indicator reflects low treatment plant efficiency in terms of removing such bacteria, and is probably related to the absence of disinfection process. Given the bacteriological results obtained, the sanitary quality of TWW at the WWTP of Baraki (Algiers) is far from acceptable for irrigation at this moment. Actually, it is suggested that the application of this studied TWW for agricultural purposes may pose a risk to workers and consumers. The disinfection of the TWW can eliminate the majority of pathogens, but this type of treatment is not practiced in the WWTP of Baraki. However, in the case of setting up an adequate disinfection system, our current results on bacteriological quality may vary and become very promising.

In this study, most of the physico-chemical parameters are in conformity with the standards for TWW reuse in irrigation. It was also noted very high levels of ammonia and TN which are higher than the permissible USEPA and FAO limits for irrigation. The concentration of heavy metals is very low compared to FAO standards and will have no toxicity effects on the soil or on the plant in the short or long term. The Wilcox diagram illustrates that most of the TWW samples fall in the field of C3-S1, indicating high salinity and low sodium water, which can be used for irrigation on almost all types of soil without danger of exchangeable sodium and may be suitable for irrigation of salt-tolerant species and well-drained soil and leached. The biological assessment revealed the absence of Salmonella spp. and Staphylococcus spp., while, the values of TC and FC were very high and exceeded the OMS guidelines. These high concentrations are certainly linked to the absence of the disinfection process. These data show that this TWW cannot be used in agriculture without adequate treatment and an investigation should be carried out to identify the origin of high levels of nitrogen and problem management. In order to minimize the health risk linked to the reuse of TWW with such bacteriological quality, we strongly recommend the establishment of an effective disinfection system in WWTP of Baraki (Algeria), periodic evaluation of the physico-chemical and bacteriological status of TWW, agricultural extension among farmers who use this type of water for the irrigation of their crops, as well as consumer awareness. To guarantee the protection of public health, it is essential to comply with strict standards and regulations adapted to the specific nature of different cultures in accordance with WHO and FAO recommendations. However, the main limitations to this type of use are psychological and cultural, associated with the perception of wastewater as dangerous and unhealthy.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.